Abstract
Most bat collisions with wind turbine blades coincide with fall migration. Previous studies showing video data of bats approaching wind turbines from downwind suggest bats may be attracted to the areas of reduced airflow (i.e., windbreaks) downwind of turbines. Because the main fatalities at wind turbines are tree-roosting bat species, and bats are sensitive to air flow, it is also possible that bats are attracted to wind turbines because they associate the wakes with tall trees and roosting or foraging opportunities. Better understanding bat behavior near wind turbines and tall trees could provide insights into potential attraction/repulsion dynamics and development of fatality minimization techniques.
In this study, we used 3–D thermal video to collect bat flight data to generate a detailed view of bat movements to better understand why bat collision events occur. We predicted that
- Wind turbines and trees would have similar wake profiles
- Bats would be most active in the wakes of wind turbines and tall trees
- The most active species would be migratory tree bats
- Bat flight direction would be toward the wind turbines.
We positioned pairs of thermal cameras adjacent to wind turbines and tall trees to obtain 3–D video of bats within the predicted wakes of these structures. This approach is novel compared to the typical observations of bats at wind turbines, which usually focus on interaction with the wind turbine structure (tower, nacelle, blades). We refer to video detections at wind turbines as “targets,” as it was not possible to differentiate bats and birds due to the small detection size of objects in the video. We also positioned pairs of cameras to capture the rotor-swept zone of wind turbines to determine if collisions were associated with particular airflow dynamics; however, these cameras had technical difficulties and were unable to record bat interactions in the rotor-swept area. We modeled the wake zones of wind turbines and tall trees using onsite weather data, and we recorded bat echolocation calls using detectors at ground level at wind turbines and trees and mounted to wind turbine nacelles. We modeled video and acoustic activity in relation to wind speed, wind direction, and barometric pressure. Our analyses suggest that bat and bird activity at wind turbines was highest on nights with northerly winds, and that animals mainly flew with the wind to the southwest, consistent with migratory behavior. Flights past wind turbines were mostly linear, and acoustic detections were mainly of migratory species, whereas flights at trees were mostly nonlinear and acoustic detections (after manual review) were mainly of nonmigratory species. Wakes at wind turbines were much larger than wakes at trees, although velocity deficits were similar. However, wake effects were minimal at trees and were less than 1 meter per second at wind turbines. Only 2% of 13,298 targets (i.e., birds and bats) observed using thermal video cameras at wind turbines occurred in the wake, and flights in the direction of the wind turbine were exceedingly rare. The low number of flying animals recorded within wind turbine wakes is surprising, given the presumed aerodynamic advantage of flying in areas of lower wind speed, which suggests that bats may either actively avoid wake regions or that air currents affect bat flight paths such that they do not enter the wake region.